Due to the huge size of the raw digital video data (or image sequences), compression must be applied such data so that they may be transmitted and stored. There have been many important video compression standards, including the ISO/IEC MPEG-1, MPEG-2, MPEG-4 standards and the ITU-T H.261, H.263, H.263+, H.263++, H.264 standards. The ISO/IEC MPEG-1/2/4 standards are used extensively by the entertainment industry to distribute movies, digital video broadcast including video compact disk or VCD (MPEG-1), digital video disk or digital versatile disk or DVD (MPEG-2), recordable DVD (MPEG-2), digital video broadcast or DVB (MPEG-2), video-on-demand or VOD (MPEG-2), high definition television or HDTV in the US (MPEG-2), etc. The later MPEG-4 was more advanced than MPEG-2 and can achieve high quality video at lower bit rate, making it very suitable for video streaming over internet, digital wireless network (e.g. 3G network), multimedia messaging service (MMS standard from 3GPP), etc. MPEG-4 will be included in the next generation DVD players. The ITU-T H.261/3/4 standards are designed for low-delay video phone and video conferencing systems. The early H.261 was designed to operate at bit rates of p*64 kbit/s, with p=1,2, . . . , 31. The later H.263 is very successful and is widely used in modern day video conferencing systems, and in video streaming in broadband and in wireless network, including the multimedia messaging service (MMS) in 2.5G and 3G networks and beyond. The latest H.264 (also called MPEG-4 Version 10, or MPEG-4 AVC) is currently the state-of-the-art video compression standard. It is so powerful that MPEG decided to jointly develop with ITU-T in the framework of the Joint Video Team (JVT). The new standard is called H.264 in ITU-T and is called MPEG-4 Advance Video Coding (MPEG-4 AVC), or MPEG-4 Version 10. Based on H.264, a related standard called the Audio Visual Standard (AVS) is currently under development in China. Other related standards may be under development.
H.264 has superior objective and subjective video quality over MPEG-1/2/4 and H.261/3. The basic encoding algorithm of H.264 is similar to H.263 or MPEG-4 except that integer 4×4 discrete cosine transform (DCT) is used instead of the traditional 8×8 DCT and there are additional features include intra prediction mode for I-frames, multiple block sizes and multiple reference frames for motion estimation/compensation, quarter pixel accuracy for motion estimation, in-loop deblocking filter, context adaptive binary arithmetic coding, etc.
In particular, H.264 allows the encoder to store five reference pictures for motion estimation (ME) and motion compensation (MC). However, the processing time increases linearly with the number of reference pictures used. This is because motion estimation needs to be performed for each reference frame in a simple implementation. This full selection process (the examination of all five reference frames) provides the best coding result but the five-fold increase in computation is very high.